1. Field of the Disclosure
The disclosure relates to the field of electromagnetic induction well logging. More specifically, the present disclosure is a method of reducing effects of conductive drill pipes on signals in transient electromagnetic measurements for evaluation of earth formations ahead of the drill bit.
2. Description of the Related Art
Electromagnetic induction resistivity instruments can be used to determine the electrical conductivity of earth formations surrounding a wellbore. An electromagnetic induction well logging instrument is described, for example, in U.S. Pat. No. 5,452,761 issued to Beard et al. The instrument described in the Beard '761 patent includes a transmitter coil and a plurality of receiver coils positioned at axially spaced apart locations along the instrument housing. An alternating current is passed through the transmitter coil. Voltages that are induced in the receiver coils as a result of alternating magnetic fields induced in the earth formations are then measured. The magnitude of certain phase components of the induced receiver voltages are related to the conductivity of the media surrounding the instrument.
Deep-looking electromagnetic tools are used to achieve a variety of different objectives. Deep-looking tools attempt to measure the reservoir properties between wells at distances ranging from tens to hundreds of meters (ultra-deep scale). There are single-well and cross-well approaches, most of which are rooted in the technologies of radar/seismic wave propagation physics. This group of tools is naturally limited by, among other things, their applicability to only high-resistivity formations and the power available downhole.
At the ultra-deep scale, technology may be employed based on transient field behavior. The transient electromagnetic field method has been used in surface geophysics. Typically, voltage or current pulses that are excited in a transmitter initiate the propagation of an electromagnetic signal in the earth formation. Electric currents diffuse outwards from the transmitter into the surrounding formation. At different times, information arrives at the measurement sensor from different investigation depths. Particularly, at a sufficiently late time, the transient electromagnetic field is sensitive mainly to remote formation zones and only slightly depends on the resistivity distribution in the vicinity of the transmitter. This transient field is especially important for logging.
The transmitter may be either a single-axis or multi-axis electromagnetic and/or electric transmitter. In one embodiment, the transient electromagnetic (TEM) transmitters and TEM receivers are separate modules that are spaced apart and interconnected by lengths of cable, with the TEM transmitter and TEM receiver modules being separated by an interval of from one meter up to 200 meters, as selected. Radial depth of investigation δ is related to time by the relation δ=√{square root over (2t/σμ)}. Thus, the depth of investigation increases with time t. Similarly, the conductivity a of the surrounding material inversely affects the depth of investigation δ. As conductivity σ increases, the radial depth of investigation decreases. Finite conductivity casing of the apparatus, therefore, can reduce the radial depth of investigation.
Rapidly emerging measurement-while-drilling (MWD) technology introduces a new, deep (3-10 meters) scale for an electromagnetic logging application related to well navigation in thick reservoirs. The major problem associated with the MWD environment is the introduction of a metal drill pipe close to the area being measured. This pipe produces a very strong response and significantly reduces the sensitivity of the measured EM field to the effects of formation resistivities and remote boundaries. Previous solutions for this problem typically comprise creating a large spacing (up to 20 meters) between transmitter and receiver. However, the sensitivity of such a tool to remote boundaries is low.
In a typical transient induction tool, current in the transmitter coil drops from an initial value I0 to 0 at the moment t=0. Subsequent measurements are taken while the rotating tool is moving along the borehole trajectory. The currents induced in the drilling pipe and in the formation (i.e., eddy currents) begin diffusing from the region close to the transmitter coil in all directions surrounding the transmitter. These currents induce electromagnetic field components that can be measured by induction coils placed along the conductive pipe. Signal contributions due to the eddy currents in the pipe are considered to be parasitic since the signal due to these eddy currents is much stronger than the signal from the formation. In order to receive a signal that is substantially unaffected by the eddy currents in the pipe, one can measure the signal at the very late stage, at a time when the signals from the formation dominate parasitic signals due to the pipe. Although the formation signal dominates at the late stage, it is also very small, and reliable measurement can be difficult. In prior methods, increasing the distance between transmitter and receivers reduces the influence of the pipe and shifts the dominant contribution of the formation to the earlier time range. Besides having limited resolution with respect to an oil/water boundary, such a system is very long (up to 10-15 m) which is not desirable and/or convenient for an MWD tool.
U.S. Pat. No. 7,150,316 to Itskovich, having the same assignee as the present disclosure and the contents of which are incorporated herein by reference, teaches an apparatus for use in a borehole in an earth formation and a method of using the apparatus. A tubular portion of the apparatus includes a damping portion for interrupting a flow of eddy currents. A transmitter positioned within the damping portion propagates a first transient electromagnetic signal in the earth formation. A receiver positioned within the damping portion axially separated from the transmitter receives a second transient electromagnetic signal indicative of resistivity properties of the earth formation. A processor determines from the first and second transient electromagnetic signals a resistivity of the earth formation. The damping portion includes at least one cut that may be longitudinal or azimuthal. A non-conductive material may be disposed within the cut. Alternatively, the damping portion may include segments having cuts and segments having a non-conducting material on an outer surface thereof.
U.S. patent application Ser. No. 11/682,381 of Itskovich having the same assignee as the present disclosure and the contents of which are incorporated herein by reference discloses a combination of electromagnetic and magnetostatic shielding to perform measurements ahead of the drill bit. It has been found that the device of Itskovich provides the ability to determine a distance to an interface in the earth formation in which the borehole is inclined at angles of less than 45° to the interface. The term “interface” is intended to include a boundary between two fluids in an earth formation and also a boundary between different layers of the earth formation. At larger inclinations, the resistivity sensor may be considered to be “looking ahead of the drill” and the ability to identify interfaces 10 m ahead of the bottomhole assembly is relatively poor. These larger angles are commonly encountered when first drilling into a reservoir. Itskovich '381 shows that a combination of electromagnetic and magnetostatic shielding provides improved results. In the present disclosure, we discuss further development of the methods of Itskovich and Itskovich '381.